HMX is a crystalline solid used as an explosive and as an ingredient of propellants. It is polymorphic, having four crystalline forms commonly knonw as alpha HMX, beta HMX, gamma HMX, and delta HMX. Beta HMX is considered to be the least impact sensitive of the four, and for that reason is the only form of HMX which meets present military specifications. For reasons of safety and efficiency, it would be desirable to crystallize HMX in the beta form exclusively.
U.S. Pat. No. 4,432,902, issued to McGuire, et al. on Feb. 21, 1984, teaches the formation of HMX by addition of dinitrogen pentoxide (N.sub.2 O.sub.5) to a solution of 1,5-diacetyl-3,7-dinitro-1,3,5,7-tetraazacyclooctane (DADN): ##STR2## or 1,3,5,7-tetraazacyclooctane (TAT) in an unspecified solvent (which is understood to be anhydrous nitric acid) to form a solution of HMX. The solution is then poured into a bath of water (in which HMX is insoluble) to cause the HMX to crystallize. While the McGuire patent uses the reaction scheme preferred herein, beta HMX is not consistently produced because the reaction conditions, particularly at the point where the HMX solution is added to water, are not precisely controlled. McGuire discloses that a concentrated acid should be added to water suddenly (Example 14) to precipitate HMX. This causes localized or general exotherms which encourage formation of undesired polymorphs of HMX. McGuire's method has been shown to be difficult to practice on a larger than laboratory scale.
An earlier example of prior art is U.S. Pat. No. 3,297,681, issued to Wright, et al. on Jan. 10, 1967. This patent describes a process for producing beta HMX. In the Wright process, crude (polymorphic) HMX is first produced in crystalline form. This step in itself is dangerous, for polymorphic or impure HMX is less stable than pure beta HMX. The crude HMX is then dissolved in an organic solvent, the solution is cooled to supersaturate it, seeded with critically sized beta HMX, and then water is added to the HMX solution suddenly to precipitate HMX. The solvent is then removed and the residue filtered to extract beta HMX. The Wright, et al. process will produce beta HMX, but the intermediate production of crude HMX is hazardous, and should be avoided. Also, the Wright, et al. process requires that the HMX in solution must be saturated before it can be seeded, since the seed crystals would dissolve in an unsaturated solution. The process thus is not useful for recrystallizing HMX from more dilute solutions. This process also suffers from the need to precisely control process conditions so that beta HMX will be quantitatively formed to the exclusion of the other polymorphs.
Several other pertinent references are U.S. Pat. Nos. 3,304,300, issued to Watters on Feb. 14, 1967; 3,676,425, issued to Dawson, et al. on July 11, 1972; 3,770,721 issued to Robbins, et al. on Nov. 6, 1973; and 4,086,228, issued to Solomon, et al. on Apr. 25, 1978; and French Pat. No. 1,463,470, issued Dec. 23, 1966, cited in Chemical Abstracts, Vol. 67 (16) 75044j.